1. Field of the Invention
The present invention relates to new oil-in-water (O/W) type emulsions, that is to say emulsions in which the continuous dispersing phase is aqueous and the discontinuous disperse phase is oily, which are both markedly acidic and completely stable, as well as to cosmetic and/or dermatological and/or pharmaceutical compositions containing such emulsions. The present invention also relates to the use of a specific polymer for the purpose of stabilizing O/W type emulsions or compositions based on such emulsions possessing markedly acidic pH values.
2. Discussion of the Background
For various reasons associated especially with the fact that they are very agreeable to use (possessing gentleness, emollience and the like), oil-in-water type emulsions are nowadays widely employed in the field of cosmetic or pharmaceutical compositions intended for topical use (skin, hair). Besides the traditional emulsifying agents which are usually needed for their preparation, these emulsions generally contain, both in the aqueous phase and in the oily phase (also known as fatty phase), cosmetically or therapeutically active principles which are required to be released on application of the composition containing the emulsion to the body.
Some oil-in-water type emulsions, while containing emulsifying agents (or surfactants) as mentioned above, are of low chemical stability, especially when these emulsions are highly concentrated with respect to the fatty phase and/or when they contain agents which are not compatible with the remainder of the constituents forming the emulsion. This lack or absence of stability manifests itself in practice in a separation (by settling) of the aqueous and oily phases of the emulsion, it being possible for this separation to be more or less gradual and/or more or less complete depending on the case.
To avoid this undesirable phenomenon, it is often necessary to resort to so-called thickening agents, which are then introduced into the emulsion and whose primary function is to create, within the aqueous phase, a gelatinous matrix serving to immobilize (or "confine") the particles (or globules) of the fatty phase in its three-dimensional network, thus providing for a kind of stabilization or mechanical maintenance of the whole of the emulsion. This approach, though effective, nevertheless substantially modifies the rheological properties of the initial emulsion, and this is not always compatible with some of the applications envisaged for the final composition.
The above stabilization technique is unfortunately not generally applicable, that is to say it is not suitable for all oil-in-water type emulsions for which stabilization might be necessary or desirable. Such is the case with emulsions which possess markedly acidic pH values, especially pH values below or equal to 3.5, and still more especially below or equal to 3, and which are intrinsically of low chemical stability. Such acidic emulsions may be encountered, in particular, when it is desired to obtain cosmetic and/or pharmaceutical compositions containing acidic active agents, such as, for example, hydroxy acids having exfoliative/moisturizing properties, acidic compositions for which there is currently a strong need.
In effect, when it is desired to stabilize such acidic emulsions (i.e. having pH values below or equal to 3.5) in a conventional manner by means of the standard thickening agents, it is found not only that these thickening agents do not exert their known primary thickening function therein, but also that they produce a destabilizing and destructive effect contrary to the desired effect, leading to a separation of the aqueous and oily phases of the initial emulsion. As far as the Applicant is aware, there is at present no simple, effective, and reliable method enabling acidic and stable oil-in-water type emulsions to be prepared and obtained.